Electroluminescent display devices

ABSTRACT

An active matrix display device comprises an array of display pixels, with each pixel comprising an EL display element, a light-dependent device for detecting the brightness of the display element and a drive transistor circuit for driving a current through the display element. The drive transistor is controlled in response to the light-dependent device output so that ageing compensation can be implemented. The light-dependent device is located laterally of the area of light emitting material of the EL display element. In this way, the light-dependent device does not cause step coverage problems and can be integrated into the pixel layout without affecting the pixel aperture. Furthermore, the light dependent device can extend alongside the full length of the area of light emitting material so that it receives light input from a large part of the display element area.

This invention relates to electroluminescent display devices,particularly active matrix display devices having an array of pixelscomprising light-emitting electroluminescent display elements and thinfilm transistors. More particularly, but not exclusively, the inventionis concerned with an active matrix electroluminescent display devicewhose pixels include light sensing elements which are responsive tolight emitted by the display elements and used in the control ofenergisation of the display elements.

Matrix display devices employing electroluminescent, light-emitting,display elements are well known. The display elements commonly compriseorganic thin film electroluminescent elements, (OLEDs), includingpolymer materials (PLEDs), or else light emitting diodes (LEDs). Theterm LED used below is intended to cover all of these possibilities.These materials typically comprise one or more layers of asemiconducting conjugated polymer sandwiched between a pair ofelectrodes, one of which is transparent and the other of which is of amaterial suitable for injecting holes or electrons into the polymerlayer.

The display elements in such display devices are current driven and aconventional, analogue, drive scheme involves supplying a controllablecurrent to the display element. Typically a current source transistor isprovided as part of the pixel configuration, with the gate voltagesupplied to the current source transistor determining the currentthrough the electroluminescent (EL) display element. A storage capacitorholds the gate voltage after the addressing phase. An example of such apixel circuit is described in EP-A-0717446.

Each pixel thus comprises the EL display element and associated drivercircuitry. The driver circuitry has an address transistor which isturned on by a row address pulse on a row conductor. When the addresstransistor is turned on, a data voltage on a column conductor can passto the remainder of the pixel. In particular, the address transistorsupplies the column conductor voltage to the current source, comprisingthe drive transistor and the storage capacitor connected to the gate ofthe drive transistor. The column, data, voltage is provided to the gateof the drive transistor and the gate is held at this voltage by thestorage capacitor even after the row address pulse has ended. The drivetransistor in this circuit is implemented as a p-channel TFT, (Thin FilmTransistor) so that the storage capacitor holds the gate-source voltagefixed. This results in a fixed source-drain current through thetransistor, which therefore provides the desired current sourceoperation of the pixel. The brightness of the EL display element isapproximately proportional to the current flowing through it.

In the above basic pixel circuit, differential ageing, or degradation,of the LED material, leading to a reduction in the brightness level of apixel for a given drive current, can give rise to variations in imagequality across a display. A display element that has been usedextensively will be much dimmer than a display element that has beenused rarely. Also, display non-uniformity problems can arise due to thevariability in the characteristics of the drive transistors,particularly the threshold voltage level.

Improved voltage-addressed pixel circuits which can compensate for theageing of the LED material and variation in transistor characteristicshave been proposed. These include a light sensing element which isresponsive to the light output of the display element and acts to leakstored charge on the storage capacitor in response to the light outputso as to control the integrated light output of the display elementduring the drive period which follows the initial addressing of thepixel. Examples of this type of pixel configuration are described indetail in WO 01/20591 and EP 1 096 466. In an example embodiment, aphotodiode in the pixel discharges the gate voltage stored on thestorage capacitor and the EL display element ceases to emit when thegate voltage on the drive transistor reaches the threshold voltage, atwhich time the storage capacitor stops discharging. The rate at whichcharge is leaked from the photodiode is a function of the displayelement output, so that the photodiode serves as a light-sensitivefeedback device.

With this arrangement, the light output from a display element isindependent of the EL display element efficiency and ageing compensationis thereby provided. Such a technique has been shown to be effective inachieving a high quality display which suffers less fromnon-uniformities over a period of time. However, this method requires ahigh instantaneous peak brightness level to achieve adequate averagebrightness from a pixel in a frame time and this is not beneficial tothe operation of the display as the LED material is likely to age morerapidly as a result.

In an alternative approach, the optical feedback system is used tochange the duty cycle with which the display element is operated. Thedisplay element is driven to a fixed brightness, and the opticalfeedback is used to trigger a transistor switch which turns off thedrive transistor rapidly. This avoids the need for high instantaneousbrightness levels, but introduces additional complexity to the pixel.

The use of optical feedback systems is considered as an effective way ofovercoming differential ageing of the LED display elements.

One problem with these compensation schemes is that they are not easilyimplemented with a top-emitting structure. The difficulty with topemission is that light cannot enter the photo-sensor in the activematrix because the anode will cover most of the pixel electronics and itwill generally be highly reflective and non-transmitting.

Another problem relates to the efficiency and implementation of theoptical feedback element. Two types of optical sensor have beenconsidered. One approach is that a low temperature polysilicon (LTPS)TFT can be used as a light sensitive element, gated with the ITO LEDanode. Alternatively, an extra transparent ITO level can be added intothe technology to provide a gate for the photo TFT instead of the LEDanode. A difficulty is that the conversion efficiency from photons toelectrons is very low in the green and red bands (e.g. 2% and 1%respectively). Therefore, large devices that fill the aperture arerequired. These large devices also present other difficulties such aslarge dark currents and high parasitic capacitance.

A second approach is to integrate an amorphous silicon PIN/NIPphotodiode with the LTPS process. This results in highly efficientoptical sensors, for example RGB efficiencies of 80%, 70% and 40%respectively. This enables very small NIP devices to be used in thepixel. However, this also means that edge non-uniformities that occurwhen defining the device will be important and will create differencesacross the display. Also, a small device will only sample a small areaof the LED pixel aperture, and this may not be representative of thewhole aperture leading to poor corrections.

A further difficulty with both approaches is that the photo-sensitivedevices conventionally sit under the aperture of the LED and due to stepcoverage problems, the photo sensors may induce non-uniformity in thepixel aperture, again leading to poor differential aging correction. Thephotodiode may create a vertical step of around 0.2-1.5 μm, and this isdifficult to planarise. Thus, even for bottom emitting structures, thelocation of the photodiode beneath the pixel layer can cause problems.

According to the invention, there is provided an active matrix displaydevice comprising an array of display pixels, each pixel comprising:

a current-driven light emitting display element comprising an area oflight emitting material sandwiched between electrodes;

a light-dependent device for detecting the brightness of the displayelement; and

a drive transistor circuit for driving a current through the displayelement, wherein the drive transistor is controlled in response to thelight-dependent device output, wherein

the light-dependent device is located laterally of the area of lightemitting material.

By locating the light-dependent device to the side of the light emittinglayer, the device does not cause step coverage problems in the lightemitting material layer. Furthermore, the position of thelight-dependent device to the side of the pixel aperture area enablesthe device to be integrated into the pixel layout without affecting thepixel aperture. Furthermore, the light dependent device can extendalongside the full length of the area of light emitting material so thatit receives light input from a large part of the display element area.

The light-dependent device preferably comprises a photodiode, forexample having a PIN or NIP diode stack and top and bottom contactterminals.

By receiving light laterally into such a structure, the efficiency ofthe light-dependent device can be improved, as losses through the top(or bottom) doped layers can be avoided, with light penetrating directlyinto the intrinsic layer.

The top contact terminal of the diode preferably extends over the top ofthe stack and down one side of the stack and acts as a light shield topixels on that side of the photodiode. In this way, the diodeconfiguration receives light laterally from one side, and providesshielding for light received laterally from the other side.

The display element electrodes may comprise a top substantiallytransparent electrode and a bottom substantially non-transparent,reflective electrode. This defines a top emitting configuration. Theinvention enables in-pixel photosensing to take place without requiringa reduction in pixel aperture in such a device.

The bottom electrode can be used not only for the display function, butalso for reflecting light from the display element to the lightdependent device. For example, the bottom electrode can reflect lightemitted at an angle to the normal greater than a first angle to thelight dependent device. Light emitted at an angle less than the firstangle is then display light, and the light at greater than the firstangle is essentially lateral illumination.

A further reflecting layer can be provided above the light dependentdevice and for reflecting reflected light from the reflecting bottomelectrode to the light dependent device. Thus, a double reflection isprovided to direct light laterally from the display element to thelight-dependent device.

The device may further comprise a plurality of printing dams, and thelight emitting material then comprises a printable material. In thiscase, the reflecting layer can be formed at the base of the printingdams. The light sensitive devices are then formed beneath the printingdams.

The printing dams may comprise an insulating body and a conducting metallayer over the insulating body. The conducting metal layer can thenprovide a lower resistance shunt connecting the top substantiallytransparent electrodes and it can also define the reflecting layer.

In another embodiment, the electrodes may comprise a top substantiallytransparent electrode and a bottom substantially transparent electrode,and the device further comprises an additional reflective layer beneaththe bottom electrode. This provides a space between the display materiallayer and the bottom reflective electrode, which enables more laterallydirected light to be captured by the light-dependent device. A topreflecting layer may again be provided above the light dependent deviceand for reflecting light from the bottom reflective layer to the lightdependent device. This top reflecting layer can be formed at the levelof the bottom electrode of the light emitting display element.

The light-dependent device can extend alongside the area of lightemitting material and can extend along substantially the full length ofone side of the area of light emitting material. It may also extendaround an upper and lower portion of the area of light emittingmaterial. This maximises the area of the light dependent device exposedto lateral light from the display element.

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 is a simplified schematic diagram of an embodiment of activematrix EL display device;

FIG. 2 illustrates a known form of pixel circuit;

FIG. 3 shows a first known optical feedback pixel design;

FIG. 4 shows a second known optical feedback pixel design;

FIG. 5 shows schematically pixels of a display device of the invention;

FIG. 6 shows a known structure of a bottom emitting display pixel;

FIG. 7 shows a known structure of a top emitting display pixel;

FIG. 8 shows a known structure of a bottom emitting display pixelincorporating a light sensitive element;

FIG. 9 shows a first example of a top emitting display pixel structureincorporating a light sensitive element in accordance with theinvention;

FIG. 10 shows a second example of a top emitting display pixel structureincorporating a light sensitive element in accordance with theinvention;

FIG. 11 shows a third example of a top emitting display pixel structureincorporating a light sensitive element in accordance with theinvention;

FIG. 12 shows a fourth example of a top emitting display pixel structureincorporating a light sensitive element in accordance with theinvention;

FIG. 13 shows one way in which the invention can be applied to a tripletof display sub-pixels;

FIG. 14 is a cross section from FIG. 13;

FIG. 15 shows a second way in which the invention can be applied to atriplet of display sub-pixels; and

FIG. 16 shows a fifth example of a top emitting display pixel structureincorporating a light sensitive element in accordance with the inventionand using a photosensitive transistor.

The same reference numbers are used throughout the Figures to denote thesame or similar parts.

FIG. 1 shows a known active matrix electroluminescent display device.The display device comprises a panel having a row and column matrixarray of regularly-spaced pixels, denoted by the blocks 1 and comprisingelectroluminescent display elements 2 together with associated switchingmeans, located at the intersections between crossing sets of row(selection) and column (data) address conductors 4 and 6. Only a fewpixels are shown in the Figure for simplicity. In practice there may beseveral hundred rows and columns of pixels. The pixels 1 are addressedvia the sets of row and column address conductors by a peripheral drivecircuit comprising a row, scanning, driver circuit 8 and a column, data,driver circuit 9 connected to the ends of the respective sets ofconductors.

The electroluminescent display element 2 comprises an organic lightemitting diode, represented here as a diode element (LED) and comprisinga pair of electrodes between which one or more active layers of organicelectroluminescent material is sandwiched. The display elements of thearray are carried together with the associated active matrix circuitryon one side of an insulating support. Either the cathodes or the anodesof the display elements are formed of transparent conductive material.The support is of transparent material such as glass and the electrodesof the display elements 2 closest to the substrate may consist of atransparent conductive material such as ITO so that light generated bythe electroluminescent layer is transmitted through these electrodes andthe support so as to be visible to a viewer at the other side of thesupport.

FIG. 2 shows in simplified schematic form the most basic pixel and drivecircuitry arrangement for providing voltage-addressed operation. Eachpixel 1 comprises the EL display element 2 and associated drivercircuitry. The driver circuitry has an address transistor 16 which isturned on by a row address pulse on the row conductor 4. When theaddress transistor 16 is turned on, a voltage on the column conductor 6can pass to the remainder of the pixel. In particular, the addresstransistor 16 supplies the column conductor voltage to a current source20, which comprises a drive transistor 22 and a storage capacitor 24.The column voltage is provided to the gate of the drive transistor 22,and the gate is held at this voltage by the storage capacitor 24 evenafter the row address pulse has ended.

The drive transistor 22 in this circuit is implemented as a p-type TFT,so that the storage capacitor 24 holds the gate-source voltage fixed.This results in a fixed source-drain current through the transistor,which therefore provides the desired current source operation of thepixel.

In the above basic pixel circuit, for circuits based on polysilicon,there are variations in the threshold voltage of the transistors due tothe statistical distribution of the polysilicon grains in the channel ofthe transistors. Polysilicon transistors are, however, fairly stableunder current and voltage stress, so that the threshold voltages remainsubstantially constant.

The variation in threshold voltage is small in amorphous silicontransistors, at least over short ranges over the substrate, but thethreshold voltage is very sensitive to voltage stress. Application ofthe high voltages above threshold needed for the drive transistor causeslarge changes in threshold voltage, which changes are dependent on theinformation content of the displayed image. There will therefore be alarge difference in the threshold voltage of an amorphous silicontransistor that is always on compared with one that is not. Thisdifferential ageing is a serious problem in LED displays driven withamorphous silicon transistors.

In addition to variations in transistor characteristics there is alsodifferential ageing in the LED itself. This is due to a reduction in theefficiency of the light emitting material after current stressing. Inmost cases, the more current and charge passed through an LED, the lowerthe efficiency.

FIGS. 3 and 4 show examples of pixel layout with optical feedback toprovide ageing compensation.

In the pixel circuit of FIG. 3, a photodiode 27 discharges the gatevoltage stored on the capacitor 24 (C_(data)), causing the brightness toreduce. The display element 2 will no longer emit when the gate voltageon the drive transistor 22 (T_(drive)) reaches the threshold voltage,and the storage capacitor 24 will then stop discharging. The rate atwhich charge is leaked from the photodiode 27 is a function of thedisplay element output, so that the photodiode 27 functions as alight-sensitive feedback device. Once the drive transistor 22 hasswitched off, the display element anode voltage reduces causing thedischarge transistor 29 (T_(discharge)) to turn on, so that theremaining charge on the storage capacitor 24 is rapidly lost and theluminance is switched off.

As the capacitor holding the gate-source voltage is discharged, thedrive current for the display element drops gradually. Thus, thebrightness tails off. This gives rise to a lower average lightintensity.

FIG. 4 shows a circuit which has been proposed by the applicant, andwhich has a constant light output and then switches off at a timedependent on the light output.

The gate-source voltage for the drive transistor 22 is again held on astorage capacitor 24 (C_(store)). However, in this circuit, thiscapacitor 24 is charged to a fixed voltage from a charging line 32, bymeans of a charging transistor 34. Thus, the drive transistor 22 isdriven to a constant level which is independent of the data input to thepixel when the display element is to be illuminated. The brightness iscontrolled by varying the duty cycle, in particular by varying the timewhen the drive transistor is turned off.

The drive transistor 22 is turned off by means of a discharge transistor36 which discharges the storage capacitor 24. When the dischargetransistor 36 is turned on, the capacitor 24 is rapidly discharged andthe drive transistor turned off.

The discharge transistor 36 is turned on when the gate voltage reaches asufficient voltage. A photodiode 27 is illuminated by the displayelement 2 and again generates a photocurrent in dependence on the lightoutput of the display element 2. This photocurrent charges a dischargecapacitor 40 (C_(data)), and at a certain point in time, the voltageacross the capacitor 40 will reach the threshold voltage of thedischarge transistor 36 and thereby switch it on. This time will dependon the charge originally stored on the capacitor 40 and on thephotocurrent, which in turn depends on the light output of the displayelement. The discharge capacitor initially stores a data voltage, sothat both the initial data and the optical feedback influence the dutycycle of the circuit.

There are many alternative implementations of pixel circuit with opticalfeedback. FIGS. 3 and 4 show p-type implementations, and there are alson-type implementations, for example for amorphous silicon transistors.

The invention will now be described generally with reference to FIG. 5.

As shown in FIG. 5, each pixel 50 has a light-dependent device 52located laterally with respect to the pixel electrode 54. The design ofthe device 52, preferably a PIN or NIP diode or Schottky diode, is toallow lateral illumination from the pixel of interest 50 a but to act asa light shield to a neighbouring pixel 50 b.

The photodiode 52 is constructed outside the pixel apertures. In someembodiments, the lateral illumination allows the NIP/PIN device to haveits top window covered with metal, so that the passage of ambient lightto the sensor can also be reduced.

The efficiency of the NIP/PIN photodiode used in this manner will begood across all wavelengths, as absorption losses in the N and P layersof the device are no longer seen as light can enter through the side ofthe device. The light level will be lower than if the device weredirectly under the aperture so a large photodiode is preferred as shownin FIG. 5. This removes the non-uniformity effects of very smalldevices.

In other embodiments, the laterally positioned photodiode can stillreceive light from above using reflecting paths.

The arrangement of the invention is particularly suitable for displaydevices which emit light through the cathode (top emission) rather thandevices that emit light through the anode (bottom emission). The reasonfor this will become apparent from the discussion below of theconventional pixel layouts for top and bottom emission.

FIG. 6 shows the known basic bottom emission structure including theactive matrix.

The device comprises a substrate 60 over which the drive transistorsemiconductor body 62 is deposited. A gate oxide dielectric layer 64covers the semiconductor body, and a top gate electrode 66 is providedover the gate dielectric layer 64.

A first insulating layer 68 (typically silicon dioxide or siliconnitride) provides spacing between the gate electrode (which typicallyalso forms row conductors) and the source and drain electrodes. Thesesource and drain electrodes are defined by a metal layer 70 over theinsulator layer 68, and the electrodes connect to the semiconductor bodythrough vias as shown.

A second insulating layer 72 (again typically silicon dioxide or siliconnitride) provides spacing between the source and drain electrodes (whichtypically also form column conductors) and the LED anode. The LED anode74 is provided over the second insulating layer 72.

In the case of a bottom emission display as shown in FIG. 6, this bottomanode needs to be at least partially transparent, and ITO is typicallyused.

The EL material 76 is formed in a well over the anode, and is preferablydeposited by printing. Separate sub-pixels are formed for the threeprimary colours, and a print dam 78 assists in the accurate printing ofthe different EL materials.

The print dam 78 enables printing of separate pixels. This dam layer isgenerally made of an insulating polymer and has a height of severalmicrons. A common cathode 80 is provided over the display, and this isreflective and at a common potential for all pixels (ground in FIG. 2).

FIG. 7 shows the basic known top emission structure including the activematrix. The structure is essentially the same as in FIG. 6, but theanode 74 a is reflective and the cathode 80 a is transmissive. Thecathode may again be formed from ITO, but may have a thin metal,combination of several metals (e.g. Bg/Ag), or silicide coating betweenthe ITO and polymer to control the barrier for electron injection. Forexample, this may be a thin 5 nm layer of Barium/20 nm layer of Silver.Protection and encapsulation layers 82 cover the display.

In a top-emission display, a transparent cathode is needed. The cathodedoes, however, have to be highly conductive, and at present highlyconductive transparent metals are not readily available. Therefore thecathode of top-emission displays comprises a (semi-) transparent layeron top of the emissive pixel part and shunted with a lower resistanceconducting (non-transparent) metal 79. By placing this highly conductivemetal 79 on top of the dam 78 as shown, there is no loss in pixelaperture.

According to the electrical characteristics of the materials, the anodemetal can be a high work function metal, and it is known to provide alayer of ITO on top of a reflective metal to achieve a high workfunction into the LED stack. In this way, the anode electrode can alsosatisfy requirements relating to the physical properties of theelectroluminescent materials, for example the wetting of the polymers.

FIG. 8 shows the integration of an amorphous silicon PIN/NIP photo-diode84 in a bottom emitting structure, in conventional manner. This type ofphoto-sensor is preferred as the amorphous silicon has high quantumefficiency for photo absorption.

This type of photo-sensor is ideal for bottom emission as the gate metalwhich is used to form the bottom electrode 86 of the diode stack screensthe photo-sensor from external light 87. An open top aperture of thediode stack, shown schematically in FIG. 8, allows in light from the LEDas shown by arrow 88.

The positioning of the sensor, beneath the anode layer is clearly notappropriate for top emission where the anode is a reflective and opaquemetal. Furthermore, the diode can give rise to step coverage problems inthe electroluminescent material layer, giving non-uniformity of pixelcharacteristics.

FIG. 9 shows in more detail a first implementation of the invention foruse in a top emission structure. Where the components in FIG. 9correspond to those in FIGS. 6 and 7, the same references are used andthe description is not repeated.

The photodiode 90 is positioned at the side of the EL material area ofthe display, and comprises an NIP/PIN stack for example of heightapproximately 1.5 μm. The photodiode is illuminated by light emittedfrom the side of the LED aperture as shown by arrow 92. The diode stackis sandwiched between top and bottom electrodes 93, 94, and in thestructure shown, the bottom electrode is formed from the source/drainmetal layer 70 and the top electrode 93 is formed from the anode metallayer 74 a. The top electrode covers the top of the diode stack so thatthe device is illuminated laterally only. The top electrode 93 alsoshields the diode stack from illumination from one side as well as fromabove.

The photodiode is constructed under the print dam 78 and therefore hasno effect upon the top emission aperture. Light from the LED must enterthe side wall of the diode, and for this purpose, the diode must have alarge height. A suitable amorphous silicon diode height is 1.5 μmalthough it may be lower, for example 200 nm-1 μm.

FIG. 9 gives example layer thickness for the insulator layer 72 abovethe source/drain and for the anode metal. These heights allow verticaladjustment of the LED layer 76 with respect to the diode stack. A diodeof 1.5 μm height is sufficient to gather light from the LED. Thinningthe anode metal can enable the diode to gather more light as the LEDwill emit upwardly, so the more of the photodiode above the plane of theelectroluminescent layer, the more light it will gather.

A thick amorphous silicon photodiode presents no extra planarisationissues because the diode is not under the LED aperture. The layer 72planarises the TFTs under the pixel aperture.

The diode should be made as long as possible, preferably the same lengthas a dimension of the aperture, to gather as much light as possible. Itswidth can be limited to a few microns because the wall width is simplyrequired to absorb red photons. This is also advantageous because thedam width is likely to be narrow in pixel designs at high resolution.

The screening of the photodiode from external light is achieved by thetop diode contact formed using the anode metal, as mentioned above. Evenlight entering the display at a very shallow angle will be refractedstrongly towards the display normal (as the materials used have a highrefractive index e.g. n>1.8) so that the top contact anode metal stillblocks this light. The diode should also be screened from light fromneighbouring pixels by making sure the metal anode making the topcontact on the diode of acts as a light block as shown in FIG. 9.

In the examples above, the photodiode is illuminated only laterally.However, it is possible to provide vertical illumination or to allowvertical and lateral illumination.

The scheme in FIG. 9 only collects light on a diode edge, and thevertical position of the photodiode must accordingly be set andcontrolled precisely.

FIG. 10 shows (more schematically) a modification in which a reflectivepath is defined between the display material 76 and the photodiode 90.This enables the photodiode to collect light over an area rather than anedge, and allows more flexibility in the vertical positioning of thephotodiode within the layer structure. FIG. 10 shows only the layersrelevant to the modification, and is a partial representation of thedevice.

In FIG. 10, the column metal 70 is used to reflect the downwardlydirected light, rather than a reflective anode. The LED anode 74 istransparent and connects to the column metal layer 70 through a via, asshown. The photodiode 90 is now positioned out of the direct line ofsight of the LED layer 76, and it is placed at the level of the gatemetal 62 instead of at the column metal level as in FIG. 9. Direct lightcollected may cause non-uniformity due to sensitivity to the precisevertical position, and the embodiment of FIG. 10 reduces thissensitivity.

In order to reflect light into the photodiode through a top surface, thereflector under the pixel should ideally be as deep as possible underthe ITO anode 74, and should also extend laterally beyond the ITO anode74 as shown in FIG. 10, to increase the angle of collection.

As shown in FIG. 10, the column metal 70 provides a first reflection ofdownwardly directed light. Light emitted at an angle to the normalgreater than a minimum will be reflected and directed essentiallylaterally and with an upward component (arrow 100). A reflecting layer102 is provided above the photodiode 90 for providing a secondreflection for this light component to the photodiode. For this purpose,the print dam polymer can be used as a mask for etching a reflectinglayer to leave the reflector 102 at the base of the printing dams 78.

This reflector 102 directs light to the top of the photodiode but alsoacts as a light shield for ambient light.

FIG. 11 shows a variation to FIG. 10, in which the top mirror 110 isadded at the level of the ITO anode 74. Again, the first reflection isprovided by the column metal layer 70. FIG. 11 also shows more clearlythe photodiode 90 formed on the gate metal layer 62, and also shows theTFT semiconductor layer 66 and gate dielectric layer 64.

The processing stage for connecting the ITO anode 74 to the column metallayer using vias can also be used to form an angled mirror 110 forhigher efficiency, as shown in FIG. 11. The other levels under the LED(such as the gate metal 62 and semiconductor layers 66) can also be usedto adjust the height of the pixel with respect to the photodiode, asnecessary to control light angles. The LED and top layers, as well asthe printing dam, are not shown in FIG. 11.

FIG. 12 shows a further modification in which the height differencebetween the two mirrors is further increased to improve light collectionefficiency. The dam polymer is again used to define the mirror 102 atthe higher level, but the gate metal layer 62 rather than the columnmetal is used to define the bottom mirror. The top mirror metal can alsobe etched under the dam 78 to provide an angled profile, and this canreduce any pixel to pixel leakage. The semiconductor layer 66 can alsobe removed from under the gate metal portion forming the bottomreflector to give increased separation, or left in place as shown. Thissemiconductor layer has high surface roughness, which propagates upthrough to the gate metal layer, and this roughness may enhance lightscattering to the shallow angles.

The example of FIG. 12, with the gate metal used as a reflector, givesgiving maximum ITO layer 74 to reflector separation. Having the topreflector as high up the structure as possible also enables light to becollected over a wider range of angles.

The pixels of a colour display are grouped into sub-pixels of differentcolours, and FIG. 13 shows for completeness a top view of a triplet ofthree active matrix LED pixels 50 (R, G, B), the dams 78 and thephoto-sensors 90 at one side of the pixel underneath the dam.

FIG. 14 shows a cross section of the combination of the pixel and dam.For good operation of the diode it is important to reduce the leakagecurrent. This can be provided for by placing isolating spacers 130 atboth sides of the diode to decrease current leakage at the sidewalls.FIG. 14 also shows a further method of reducing optical crosstalk, inwhich the cathode shunt metal 79 on top of the dam is asymmetric. At theright side of the dam, the metal 79 shields the photo-sensor from lightemitted by the neighbouring pixel, and this can be used in combinationwith the use of the top electrode as discussed above (for example asshown by contact 132).

The area of the photodiode that receives light is given by the area ofthe sidewall facing the LED layer. Since the height of the diode stackmay be 1.5 μm or less, the area may be very small. As mentioned above,although the height of the diodes is preferably relatively low, thewidth can be as large as the pixel length, as shown in FIG. 13.

The length of the exposed side wall of the diode stack can be increasedfurther as shown in FIG. 15. In this case, most of the circumference ofthe pixel active area is used to illuminate a photodiode, excluding oneedge (the left edge in FIG. 15). At this edge, the photodiode of theneighbouring pixel is positioned.

This configuration increases further the amount of light sensed by thephoto diode.

The examples above all use photodiode light sensors. An amorphoussilicon photo TFT can also be used, as shown in FIG. 16. This consistsof an amorphous silicon layer 150 on top of a source 152 and drain 154electrode. Photons absorbed in the channel between source and draingenerate a photocurrent which can be sensed by the source and drainelectrodes. The photocurrent can also be influenced by application of agate electrode on top of the amorphous silicon layer.

The metal dam can in this case be used both as the gate of the amorphoussilicon photo TFT and the shunt 79 between cathodes. Light emitted at aslight angle to the substrate may again be reflected by the inside ofthe metal 79 towards the photo TFT, increasing the size of thephotocurrent. The embodiment of FIG. 15 uses a dam formed from aninsulating transparent material covered by the shunt metal 79.

A low temperature polysilicon photo TFT can also be used as thephotosensitive device, with a resulting geometry similar to FIG. 16.

Display devices of the invention will find particular application asflat panel displays in mobile applications (Phone, PDA, digital camera),in (laptop) monitors, and in televisions. The invention also provides asolution to avoid ambient light sensitivity in trans-emissive displays,for example for use in window or head mounted displays. In the case of atrans-emissive display, daylight from the other side can be shielded forexample by the power supply line.

The processes involved in the manufacture of the display devices of theinvention have not been described in this application, as they will beconventional and routine to those skilled in the art. Amorphous silicon,polysilicon, microcrystalline silicon or other semiconductor transistortechnologies may be employed. The invention can be applied to any pixelcircuit in which a photosensitive device is used as a feedback elementfor each pixel.

As explained above, the invention provides particular advantages for topemission device structures. However, the invention can also be used toimprove light collection efficiency and remove step coverage problems inbottom emission display structures.

From reading the present disclosure, other modifications will beapparent to persons skilled in the art.

1. An active matrix display device comprising a plurality of printingdams and an array of display pixels, each pixel comprising: acurrent-driven light emitting display element comprising an area oflight emitting material sandwiched between electrodes; a light-dependentdevice for detecting the brightness of the display element, wherein thelight-dependent device is located laterally outside of the area of thelight emitting material defined by the vertical planar edges of thelight emitting layer of the light emitting material, and separated fromthe light emitting material by at least one insulating layer, whereinthe vertical planar edges of the light emitting material are defined ina direction between a top and a bottom electrode of the light-dependentdevice wherein the light dependent device is located in the samehorizontal plane as the light emitting material of the light emittingdisplay element and is configured to enclose the light emitting materialon at least three sides, and wherein the light dependent device isdirectly illuminated from light emitted from a side face of the lightemitting display element and travels in a horizontal plane from saidlight emitting display to said light dependent device, wherein thelight-dependent device is formed beneath one of said plurality ofprinting dams, a drive transistor circuit for driving a current throughthe display element, wherein the drive transistor is controlled inresponse to the light-dependent device output.
 2. A device as claimed inclaim 1, wherein the light-dependent device comprises a photodiode.
 3. Adevice as claimed in claim 2, wherein the photodiode comprises a PIN orNIP diode stack or a Schottky diode and top and bottom contactterminals.
 4. A device as claimed in claim 3, wherein the top contactterminal extends over the top of the stack and down one side of thestack and acts as a light shield to pixels on the one side of thephotodiode.
 5. A device as claimed in claim 1, wherein the electrodescomprise a top substantially transparent electrode and a bottomsubstantially non-transparent, reflective electrode.
 6. A device asclaimed claim 5, wherein the bottom electrode is for reflecting lightfrom the display element to the light dependent device.
 7. A device asclaimed in claim 6, wherein the bottom electrode is for reflecting lightemitted at an angle great enough to reach the light dependent device. 8.A device as claimed in claim 6, further comprising a reflecting layerabove the light dependent device and for reflecting light from thebottom electrode to the light dependent device.
 9. A device as claimedin claim 8, wherein the light emitting material comprises a printablematerial.
 10. A device as claimed in claim 9, wherein the reflectinglayer is formed at the base of the printing dams.
 11. A device asclaimed in claim 9, wherein the printing dams comprise an insulatingbody and a conducting metal layer over the insulating body.
 12. A deviceas claimed in claim 11, wherein the conducting metal layer provides alow resistance shunt connecting the top substantially transparentelectrodes.
 13. A device as claimed in claim 11, wherein the conductingmetal layer defines the reflecting layer.
 14. A device as claimed inclaim 1, wherein the electrodes comprise a top substantially transparentelectrode and a bottom substantially transparent electrode.
 15. A deviceas claimed in claim 14, wherein the device further comprises anadditional reflective layer beneath the bottom electrode.
 16. A deviceas claimed in claim 15, further comprising a reflecting layer above thelight dependent device and for reflecting light from the reflectinglayer to the light dependent device.
 17. A device as claimed in claim16, wherein the light emitting material comprises a printable material.18. A device as claimed in claim 17, wherein the reflecting layer isformed at the base of the printing dams.
 19. A device as claimed inclaim 1, wherein the light-dependent device extends alongside the areaof light emitting material and extends along substantially the fulllength of one side of the area of light emitting material.
 20. A deviceas claimed in claim 19, wherein the light-dependent device extendsaround an upper and lower portion of the area of light emittingmaterial.
 21. A device as claimed in claim 1, wherein the light emittingdisplay element comprises an electroluminescent display element.